Photographic images may be printed onto photographic film with great ease. All that is required is an optical path and unexposed film. In contrast with this, printing onto with a finite number of inks or wax transfers requires a technique that allows for gradation of shades with a single ink color. Each ink gives only the possibility of being applied (full intensity) or not being applied (zero intensity).
Perhaps the most common technique for allowing gradation of shades is the halftone system. In the halftone system, to emulate a gray midway between black and white, a small number of dots is printed on a white background. The ratio of the area covered by the dots to the area left uncovered determines how gray is the area.
Referring now to FIG. 1, a diagram of a photographic film based halftone system is shown. Other halftone systems may utilize digital techniques, but the outcome is approximately the same. In the FIG. 1 example, an original print 100 has medium gray area 102, light gray area 106, dark gray area 104, white area 110, and black area 108. The original print 100 is projected via lens 120 onto a contact screen 124 and finally onto a film negative 142. The contact screen is a precision piece of film with a pattern of transparent areas 126 alternating in a grid with opaque areas 128. The contact screen is so constructed that the opacity gradually changes from total in opaque areas 128 to not opaque in transparent areas 126.
In a controlled exposure, light coming from a white area 110 is intense enough that the entire area 138 is exposed (blackened on a negative). But with no light coming from a black area 108 the area 140 is not exposed at all (stays white on a negative). In a medium gray area 102, the reflected light is strong enough to expose the film negative 136 in much of the area adjacent to the corresponding transparent areas of contact screen 124 but not strong enough to expose the film negative 142 in the area adjacent to the corresponding opaque areas of contact screen 124. This yields an area of medium-sized dots 132. For the light gray area 106 the exposure yields an area of larger dots 134, and for the dark gray area 104 the exposure yields an area of smaller dots 136.
The film negative 142 may be used as a printing master, with the black areas etched so that only the white areas can carry the ink. When viewed from a suitable distance, an image printed from film negative 142 has a corresponding replica of the original print 100. When viewed from a suitable distance, the replica will look like the original print 100 to a viewer.
The FIG. 1 discussion was concerned with traditional optics and photographic film. A similar process may be performed with digital technology, wherein the contact screen is replaced by a digital sampling technique. The resulting dotted image may be used to generate a printing master in an analogous manner.
Referring now to FIG. 2, a diagram showing screen angles in a color halftone system is shown. The FIG. 1 discussion was for a black and white image. When printing color images, additional complications arise. Printing in color requires the use of the three subtractive primary colors, cyan (C), magenta (M), and yellow (Y). To achieve the best results, a fourth ink, black (K) is added to the CMY palate, creating a CMYK palate.
Four separate exposures, under different colored lights, create four separate film negatives for use as printing masters. However, printing inks are somewhat opaque, and if the same orientation of the contact screen is used for each color, what you will see is a print with the dots stacked one on top of the other. The dot printed last would be most visible. Therefore, the contact screen is rotated by an angle prior to the exposures that create the C film negative, the M film negative, the Y film negative, and the K film negative. The best results occur when there is a 30 degree angle between the exposures. However, with four colors this is not possible. A best compromise has been found where the C, M, and K exposures are made with the contact screen rotated by 30 degrees between exposures, and by making the Y exposure with the contact screen midway between the C and M angles. One such possibility is shown in FIG. 2. When the corresponding film negatives are converted into printing masters, a four-color CMYK halftone print may be printed.
Problems arise when the resulting CMYK halftone print is itself a candidate for subsequent scanning. When the resulting scanned image is used for either printing or direct electronic use, such as posting the image on a web page, the halftone nature of the source image will cause poor quality in either of the resulting images. Such subsequent scanning and printing or other use may take place in a computer system with a scanner and laser printer, or may take place in a digital photocopier. In each case complications arise because the CMYK halftone print has already gone through the halftone process. After scanning, subsequent attempts with screening processes may produce a poor subsequent halftone print, with blurry areas and the possibility of Moire effects. These effects are well-known to anyone who has attempted to make a photocopy of a halftone print. Similarly, after scanning, subsequent use as an electronic image on a web page will exhibit poor image quality.
Prior art techniques for avoiding these problems have been limited by a requirement of either knowing detailed information about how the source halftone print was produced, or by using an inefficient trial-and-error method. The detailed information required may include the screen frequency (inversely proportional to the distance between transparent areas in a contact screen), screen angles (such as shown in FIG. 2), the shape of the halftone dots, and many other parameters. It is highly unlikely that a copy machine operator would know the values of these parameters or even what they are. There are currently about 20 halftone screening procedures in common use, and each requires a specific procedure to undo the effects of the halftone screening. The large number of these procedures may make their use on a trial-and-error basis very time consuming.